dashboard A/C on the fritz

I think that the ac system is possibly low on refrigerant and the unit is acting like it is. When the system is first started, the low side will have about the same pressure as the high side until it is run for awhile and the low side will continue to drop in pressure until approximately 20 psi at which point a low pressure cut out switch will disengage the clutch on the compressor to protect it from damage. Allowed to sit for awhile, the low side pressure goes up past the low pressure cutoff point and the unit will work again. A set of refrigerant gauges will confirm the state of charge or, if there is a sight glass in the system, look for bubbles. If the system is low on refrigerant, be sure you know what you are doing before attempting to recharge it.